Conventionally, a generally known apparatus for driving a load in a construction machine or the like includes a hydraulic actuator connected to the load, a hydraulic pump which discharges hydraulic fluid for causing the hydraulic actuator to move, and a control valve interposed between the hydraulic pump and the hydraulic actuator. This control valve controls supply and discharge of hydraulic fluid from the hydraulic pump to the hydraulic actuator. This apparatus is based on so-called an open circuit in which hydraulic fluid contained in a tank is sucked by the hydraulic pump and supplied to the hydraulic actuator through the control valve, and the hydraulic fluid discharged from the hydraulic actuator is returned to the tank through the control valve.
In contrast to this open circuit type apparatus, Japanese Unexamined Patent Application Publication No. 2013-117098, namely, Patent Literature 1, discloses a so-called closed circuit type of hydraulic drive apparatus. The apparatus includes a variable displacement type hydraulic pump and a hydraulic actuator. The hydraulic pump and the hydraulic actuator are connected to each other so as to configure a closed circuit in which hydraulic fluid discharged from the hydraulic pump moves the hydraulic actuator while circulating in the closed circuit. The apparatus, involving a control of a speed of the hydraulic actuator by regulation of a displacement volume or rotational speed of the hydraulic pump, does not require a control valve as described above. The apparatus, thus, has an advantage of involving no motive power loss caused by pressure loss in the control valve to save energy.
There exists a hydraulic drive apparatus required to actuate a load in a direction in which gravity acts on the load, namely, a lowering direction, such as a hydraulic winch provided in a crane or an apparatus for actuating a boom or an arm in a hydraulic excavator. Regarding such a drive in the lowering direction, it is an important issue to recover kinetic energy and potential energy of the load, that is, efficiently regenerate, while applying an appropriate brake force to a falling load and a hydraulic actuator connected to the load to control a speed thereof in the lowering direction to an appropriate speed.
However, in the case of driving a hydraulic actuator, which is required to perform such a lowering direction drive, in a so-called closed circuit apparatus as described above, no means has been presented for efficiently recovering kinetic energy and potential energy of the load while appropriately controlling a speed in the lowering direction.
Patent Literature 1 discloses a technique, based on the premise that the hydraulic actuator is a variable displacement type hydraulic motor, of regulating the displacement volume of the variable displacement type hydraulic motor to thereby control a brake torque. However, this technique, depending on a capability of varying displacement of the hydraulic motor, is not applicable to the case where the hydraulic actuator has no capability of varying displacement as in the case where the hydraulic actuator is a hydraulic cylinder or a fixed displacement type hydraulic pump.